In a permanent magnet generator the magnets are typically bonded, for example glued, to the rotor core. Under normal operation conditions significant temperature increases in a generator are possible. Such temperature increases can weaken the bond between the rotor and the magnets. Unless measures are taken to keep the magnets in place, they might become detached from the rotor which could lead to serious damage to the generator.
In the document EP 1 367 700 A2 a method for manufacturing an encapsulated rotor of a permanent magnet motor is described. The method essentially uses Vacuum Assisted Resin Transfer Moulding. The described rotor is provided with rather complicated means for retaining the magnets besides the resin. The magnetic pole pieces are especially affixed to the rotor inner ring by pole retainers held by removable seal members such as screws.
In the U.S. Pat. No. 4,954,736 a method is described whereby the magnets are retained by a combination of mechanical retainers and thermosetting resin. A yoke attached to the rotor shaft is provided with a number of projections designed to hold the magnets. The projections and the magnets are dimensioned in such a way that there are small gaps between the projections and the magnets. These gaps are subsequently filled with resin in order to fix the magnets in the desired position.
The methods described in the U.S. Pat. Nos. 5,040,286 and 5,175,461 involve forcing the magnets against the rotor core by means of a cylindrical stainless steel shell, which is cold-pressed over the magnets, and filling any remaining cavities with an adhesive.